Methyl tert-butyl ether is finding increasing use as a blending component in high octane gasoline as the current gasoline additives based on lead and manganese are phased out. Currently most commercial processes for the manufacture of methyl tert-butyl ether are based upon the liquid-phase reaction of isobutylene and methanol (Eq. 1), catalyzed by a cationic ion-exchange resin (see, for example: Hydrocarbon Processing, October 1984, p. 63; Oil and Gas J., Jan. 1, 1979, p. 76; Chem. Economics Handbook-SRI, September 1986, p. 543-7051P). The cationic ion-exchange resins used in MTBE synthesis normally have the sulfonic acid functionality (see: J. Tejero, J. Mol. Catal., 42 (1987) 257; C. Subramamam et al., Can. J. Chem. Eng., 65 (1987) 613). ##STR1##
The dehydration reaction can be represented by: ##STR2##
With the expanding use of MTBE as an acceptable gasoline additive, a growing problem is the availability of raw materials. Historically, the critical raw material has been isobutylene (Oil and Gas J., Jun. 8, 1987, p. 55), however, recently, U.S. Pat. Nos. 5,099,072, 5,169,592 and 5,081,318, inter alia, assigned to Texaco Chemical Inc., disclose a one-step method for producing methyl tert-butyl ether (MTBE) from t-butanol (tBA) over various catalysts. It would be advantageous to obtain additional conversion of the t-butanol in the crude feedstock without having to recycle unconverted tertiary butanol.
"Preparation of Methyl Tert-Butyl Ether (MTBE) Over Zeolite Catalysts" is an article by Pochen Chu and Gunther H. Kuhl, Ind. Eng. Chem. Res., 26, 365, 1987. Chu et al. disclose work which identifies ZSM-5 and ZSM-11 to be selective for the preparation of MTBE. Compared to the conventional Amberlyst 15 resin, the zeolites are thermally stable, and give no acid effluent; they provide high selectivity to MTBE with little or no diisobutene yield, are less sensitive to the CH.sub.3 OH/i-C.sub.4 H.sub.8 ratio and exhibit good selectivity even at ratios approaching unity. They provide high MTBE output, despite the unfavorable thermodynamic equilibrium, since the process utilizing these zeolites can be operated at high temperature and high space velocity. In addition, deactivation is not observed in the present short catalytic tests and reactivation is not required. The excellent selectivity of these two zeolites is believed to be effected by the size of their pore structure, which provides easy access to methanol and restricted access to isobutene. Zeolite beta was also tested, giving the poorest results and small pore zeolites were inactive. As expected, large pore zeolites did not exhibit shape selectivity.
An extensive body of knowledge of zeolite properties and catalytic potential has developed in recent years. Different types of zeolites are known in the art, including natural and synthetic zeolites. Research has opened up a spectrum of new opportunities in the field of molecular shape selective catalysis, where the intracrystalline space accessible to molecules has dimensions near those of the molecules themselves. This field is discussed in an article titled "Molecular Shape Selective Catalysis", P. B. Weisz, New Horizons in Catalysis, Part A, 1980. For example, it is possible to catalyze the dehydration of n-butanol over a Linde 5 .ANG. zeolite without reacting isobutanol which may be present. Such research has led to the concept of molecular engineering.
Another good reference for familiarization with the relationship between molecular shapes, structures of zeolites and selectivity for certain catalysis is an article titled "Industrial Application of Shape Selective Catalysis", N. Y. Chen et al., Catal. Rev.-Sci. Eng., 28 185 (1986).
The zeolites of interest for shape-selective catalysis may be divided into three major groups according to their pore/channel systems. The first group includes 8-membered oxygen ring systems such as, for example, Linde A, erionite, chabazite, zeolite alpha, ZK-4, ZK-21, ZK-22 and several other less common natural zeolites.
The second group includes 10-membered oxygen ring systems such as, for example, ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-48 and laumontite, which has a puckered 10-membered oxygen ring. The rest of the medium pore zeolites are usually synthetic in origin; they are sometimes known as pentasils. They have a predominance of silicon.
The third group of zeolites is those having dual pore systems which have interconnecting channels of 12- and 8-membered oxygen ring openings. Examples include mordenite, offretite, clinoptilolite, ferrierite, etc.
In an article titled "Clays, Zeolites and other Microporous Solids for Organic Synthesis," by John M. Thomas et al., in Modern Synthetic Methods, 1989, Vol. 5, p. 249, it is stated at page 263 that the valency or the size of the exchangeable cation can be adjusted, thus fine-tuning the molecular sieving and shape-selective properties. For example, in the Na.sup.+ form of zeolite-alpha, the effective void space within the zeolite can be enlarged by replacement of Na.sup.+ by Ca.sup.2+ ions.
Dealumination of a zeolite can enhance the meso-porosity of a zeolite by increasing the Si/Al ratio of the anionic framework and can be represented by the following: ##STR3##
Hydrothermal treatment will result in ultra-stabilization of the zeolite and recent work indicates Si:Al ratios can be increased to greater than 1000.
In addition to Y-zeolites, silicoaluminophosphates (SAPOs) are useful in the instant invention. SAPOs are one type of new materials to which increasing attention has been devoted recently. These materials include: aluminophosphate-based molecular sieves (AlPO.sub.4 -n), silicoaluminophosphates (SAPO-n), and metal-containing aluminophosphates (MeAPO-n and MeAPSO-n).
The AlPO.sub.4 molecular sieves exhibit an invariant framework composition with an Al/P atomic ratio of 1 and a wide structural diversity. Their product composition expressed as an oxide formula, is xR.Al.sub.2 O.sub.3.1.0.-+.0.2P.sub.2 O.sub.5.yH.sub.2 O, where R is an amine or quaternary ammonium template, and x, and y represent the amounts needed to fill the microporous voids. Upon calcination at temperatures of 773-873 K, the molecular sieves are expressed as AlPO.sub.4 or a TO2 formula of (Al.sub.0.50 P.sub.0.50)O.sub.2. The microporous AlPO.sub.4 materials have novel structures, 5, 11, 14, 16, 18, 31, and 33. For example, the as-synthesized ALPO.sub.4 -11 has a typical composition of 1.0 Pr.sub.2 NH:1.0 Al.sub.2 O.sub.3 :1.0 P.sub.2 O.sub.5 : 4O H.sub.2 and consists of an open framework containing unidimensional 10-ring channels (3.9.times.6.3 .ANG.).
The major structures crystallized in the new generations of AlPO.sub.4 -based molecular sieves include at least 30 stable three-dimensional novel structures. Some are topological analogs of zeolites such as faujasite, A, chabazite and erionite.
The materials are classified into binary, ternary, and quaternary compositions based on the number of elements contained in the cationic framework sites of any given structure. Classes of these materials comprise compositions crystallized in the AlPO.sub.4, silicoaluminophosphates (SAPO), metal aluminophosphates (MeAPO) and non-metal element incorporated aluminophosphates (ElAPO) families.
SAPO molecular sieves were first reported by Lok et al. (U.S. Pat. No. 4,440,871). SAPO are defined as silicoaluminophosphates with the following general chemical formula EQU nR.(Si.sub.x Al.sub.y P.sub.z)O.sub.2.bH.sub.2 O
They involve a three-dimensional arrangement of SiO.sub.4, PO.sub.4, and AlO.sub.4 tetrahedral connected through shared oxygen atoms. This arrangement results in an open structure containing channels and cages with near atomic dimensions. The structures include large-pore (7-8 .ANG.), medium-pore (.about.6 .ANG.) and small pore (3-4 .ANG.) materials. The silicoaluminophosphates (SAPO) are made by the substitution of silicon for phosphorus and with some substitution of two silicons for an aluminum plus phosphorus into a hypothetical aluminophosphate framework. The general formula of the anhydrous SAPO composition is 0-0.3R (Si.sub.x Al.sub.y P.sub.z)O.sub.2 where the mole fraction of silicon, x, typically varies from 0.04 to 0.20 depending on synthesis conditions and structure type. The typical SAPO-11 framework composition is (Si.sub.0.14 Al.sub.0.44 P.sub.0.42)O.sub.2 with (x+z) greater than y.
More than 40 distinct species of zeolite materials have been identified and there are at least 130 synthetic species. The pore sizes and compositions of typical commercially available zeolites are shown in Table A.
TABLE A __________________________________________________________________________ COMMERCIALLY AVAILABLE ZEOLITES Composition Sorportion Capacity (wt %) Zeolite Pore Size/nm Si/Al Cation H.sub.2 O nC.sub.6 H.sub.14 C.sub.6 H.sub.12 __________________________________________________________________________ Faujasite x 0.74 1-1.5 Na 28 14.5 16.6 Y 0.74 1.5-3 Na 26 18.1 19.5 US-Y 0.74 &gt;3 H 11 15.8 18.3 A 0.3 1.0 K,Na 22 0 0 A 0.4 1.0 Na 23 0 0 A 0.45 1.0 Ca,Na 23 12.5 0 Chabazite 0.4 4 * 15 6.7 1 Clinoptilite 0.4 .times. 0.5 5.5 * 10 1.8 0 Erionite 0.38 4 * 9 2.4 0 Ferrierite 0.55 .times. 0.48 5-10 H 10 2.1 1.3 L-type 0.6 3-3.5 K 12 8 7.4 Mazzite 0.58 3.4 Na,H 11 4.3 4.1 Mordenite 0.6 .times. 0.7 5.5 * 6 2.1 2.1 Mordenite 0.6 .times. 0.7 5-6 Na 14 4.0 4.5 Mordenite 0.6 .times. 0.7 5-10 H 12 4.2 7.5 Offretite 0.58 4 K,H 13 5.7 2.0 Phillipsite 0.3 2 * 15 1.3 0 Silicalite 0.55 ** H 1 10.1 ZSM-5 0.55 10-500 H 4 12.4 5.9 __________________________________________________________________________ *Denotes a mineral zeolite: cations variable, but usually Na, K, Ca, Mg **Very large Si:Al
Another important aspect regarding zeolites involves methods of generating acidity. Several ways of introducing acidity into a zeolite are known in the art and they result in the formation of Bronsted acid sites. The total acidity of a zeolite catalyst depends on both the concentration of acidic sites and the strength of the individual sites. The number and nature of active sites in a zeolite catalyst can be determined in several ways, including .sup.27 Al solid state NMR, uptake of base and poisoning experiments. Maximum overall acidity is often found for Si/Al ratios in the range of 5 to 20. Bronsted acid sites formed by various methods can form Lewis acid sites by dehydroxylation: ##STR4##
Where shape selectivity by size exclusion is the key to zeolite function, it can be accomplished either through reactant selectivity or product selectivity. It is believed Columbic field effects may also play a part. Another phenomenon which has been observed to contribute is configurational diffusion which occurs in situations where structural dimensions of the catalyst approach those of molecules; even subtle changes in dimensions of molecules can result in large changes in diffusivity, see Chen et al., Catal. Rev.-Sci. Eng., supra, p. 198.
Another type of selectivity which has been observed is spatiospecificity or restricted transition state, where both the reactant molecule and the product molecule are small enough to diffuse through channels, but the reaction intermediates are larger than either the reactants or the products and are spatially constrained.
Reactants which are of particular interest in shape selective catalysis include straight-chain and slightly branched paraffins and olefins, naphthenes and aromatics.
In U.S. Pat. No. 4,943,545, to Chang et al., there is suggested modification of Y-zeolites having Si:Al ratio of at least 4 with a very dilute (0.001.fwdarw.0.1N) solution of HF in a cracking catalyst as a means of reactivation.
In U.S. Pat. No. 5,300,697 there is disclosed a catalyst for synthesis of MTBE from t-BuOH comprising hydrogen fluoride modified zeolites.
In U.S. Pat. No. 5,220,078, there is disclosed a catalyst for MTBE synthesis comprising fluorophosphoric acid-modified zeolites.
In the art, where the feedstock for producing MTBE is t-butanol, the conversions are not as high as would be desirable.
It would be a distinct advance in the art if there were a catalyst available which could withstand sustained operating temperatures of greater than 200.degree. C. and which could be used in a second stage reactor to improve the percentage of conversion of t-butanol to methyl tertiary butyl ether without the necessity of recycling.
It would be a substantial advance in the art if such a catalyst made it possible to obtain greater than 90% conversion of t-butanol, crude t-butanol/methanol feedstocks at low energy cost.